After its introduction to Europe, Anguillicola crassus, a natural parasite of the Japanese eel, had to face novel conditions that have acted as different selective forces leading to the onset of new phenotypic modifications and genetic adaptations of the parasite. This study revealed that the changes in infectivity, development, reproductive output and morphology observed in the parasite after colonization of the European eel were induced by both genetic adaptation and phenotypic plasticity.